Fluorine-containing elastomer composition

ABSTRACT

A peroxide-curable fluorine-containing elastomer composition comprising 100 parts by weight of a fluorine-containing block polymer comprising at least one elastomeric polymer chain segment and at least one non-elastomeric polymer chain segment in which the content of said non-elastomeric polymer chain segment is less than 5% by weight, 0.05 to 10 parts by weight, preferably 0.3 to 5 parts by weight, more preferably 1 to 5 parts by weight, of an organic peroxide, and 0.1 to 10 parts by weight, preferably 0.5 to 8 parts by weight, more preferably 0.5 to 5 parts by weight, of a polyfunctional co-crosslinking agent. This composition has improved processing properties during curing and molding, while it has good tensile properties, heat resistance, oil resistance and high temperature sealing properties comparable to those of conventional peroxide-curable fluorine-containing elastomer compositions.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/JP98/02269 which has an Internationalfiling date of May 25, 1998, which designated the United States ofAmerica.

FIELD OF THE INVENTION

The present invention relates to a fluorine-containing elastomercomposition. In particular, the present invention relates to aperoxide-curable fluorine-containing elastomer composition, which hasimproved molding and processing properties, while it has good tensileproperties, heat resistance, oil resistance and high temperature sealingproperties comparable to those of known peroxide-curablefluorine-containing elastomer compositions.

BACKGROUND ART

Fluorine-containing elastomer compositions are used as materials ofO-rings, gaskets, oil seals, diaphragms, hoses, rolls, seat members,etc. in various industrial fields such as automobile, ships, aircraft,hydraulic machines and general machine industries as well asenvironmental pollution-related fields. In particular, peroxide-curablefluorine-containing elastomers can be cured when fluororubbers have highfluorine concentrations, unlike the polyol curing or polyamine curingsystem. Furthermore, fluororubbers having iodine atoms at polymer chainends have an advantage that they can be cured with peroxides withoutcompounding metal oxides.

However, although peroxide-curable fluorine-containing elastomers havesuch good properties, they have low mold releasing properties andanticontamination of molds in the curing and molding steps. Thus, it isdifficult to improve the working efficiency, and also the productivitytends to deteriorate due to the decrease of yields by cracking orbreakage, when articles having complicated shapes are molded. Thus,external mold release. agents or internal mold release agents are usedto improve the mold releasing properties of fluorine-containingelastomers. However, when internal mold release agents are used, aproblem arises, that the physical properties of cured productsdeteriorate. When external mold release agents are used, the effect toimprove the mold releasing properties decreases as the number of moldingcycles increases. Therefore, it is necessary to repeatedly apply theexternal mold release agents to a mold, and further the commercial valueof products may decrease since marks of the external release-agentsremain on the surfaces of molded articles.

In the case of peroxide curing, burrs formed in the course ofcompression molding, in particular, parts of burrs which are in contactwith an air, are not sufficiently cured, and thus stain the surfaces ofmolds or molded articles.

Various improvements have been proposed to solve the above problems ofperoxide-curable fluorine-containing elastomer compositions, but theyhave their own drawbacks.

For example, fluorine-containing block polymers, which are disclosed inJP-A-59-40066 relating to a sealing material resistant to lubricant oilsand JP-B-61-49327 relating to a fluorine-containing segmented polymer,contain 5 wt. % or more of non-elastomeric polymer chain segments, andthus they have drawbacks that properties greatly deteriorate when theyare formulated in the form of fluororubber compositions and cured.

A non-tacky fluororubber composition, which is prepared by a methoddisclosed in JP-A-8-176388 comprising mixing a fluororubber and afluorine-containing block polymer, can achieve effects to some extent.However, it has-a drawback that the content of a non-elastomericcomponent in the used fluororubbery thermoplastic elastomer is high, andthus it is not easy to mix the fluororubber and the fluorine-containingblock polymer.

SUMMARY OF THE INVENTION

One object of the present invention is to solve the problems ordrawbacks of the peroxide-curable fluorine-containing elastomercompositions disclosed in the above prior arts without deterioratingother good properties of the compositions.

According to the present invention, this object can be accomplished by aperoxide-curable fluorine-containing elastomer composition comprising

(a) 100 parts by weight of a fluorine-containing block polymercomprising at least one elastomeric polymer chain segment and at leastone non-elastomeric polymer chain segment in which the content of saidnon-elastomeric polymer chain segment is less than 5% by weight,

(b) 0.05 to 10 parts by weight, preferably 0.3 to 5 parts by weight,more preferably 1 to 5 parts by weight, of an organic peroxide, and

(c) 0.1 to 10 parts by weight, preferably 0.5 to 8 parts by weight, morepreferably 0.5 to 5 parts by weight, of a polyfunctional co-crosslinkingagent.

DETAILED DESCRIPTION OF THE INVENTION

The components contained in the composition of the present inventionwill be explained.

(a) Particularly preferred examples of the fluorine-containing blockpolymer used according to the-present invention are fluorine-containingblock polymers each comprising a chain consisting of two orthree-polymer chain segments, an iodine atom present at one end of thechain, and a residue that is present at the other end of the chain andderived from an iodide compound by removing at least one iodine atomtherefrom, in which one of the polymer chain segments (when the chainconsists of two polymer chain segments), or two of the polymer chainsegments (when the chain consists three polymer chain segments) is orare one or two elastomeric polymer chain segment(s) having a molecularweight of 30,000 to 1,200,000 selected from the group consisting of:

(1) a vinylidene fluoride-(hexafluoropropylene or.pentafluoropropylene)-tetrafluoroethylene polymer (molarratio=45-90:5-50:0-35), in particular, a vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene polymer (molarratio=40-90:10-50:0-30),

(2) a perfluoro(C₁-C₃ alkyl vinyl ether) (including one having aplurality of ether bonds)-tetrafluoroethylene-vinylidene fluoridepolymer (molar ratio=15-75:0-85:0-85), in particular, a perfluoro(C₁-C₃alkyl vinyl ether)-tetrafluoroethylene (molar ratio=15-60:40-85), and

(3) a tetrafluoroethylene-propylene-comonomer (e.g. ethylene,isobutyrene, acrylic acid and its esters, methacrylic acid and itsesters, vinyl fluoride, hexafluoropropylene, vinylidene fluoride,chloroethyl vinyl ether, chlorotrifluoroethylene, or perfluoro (C₁-C₃alkyl vinyl ether)) polymer (molar ratio=40-70:30-60:0-20), and theremaining polymer chain segment is a non-elastomeric polymer chainsegment selected from the group consisting of:

(4) a vinylidene fluorine-tetrafluoroethylene polymer (molarratio=0-100:0-100), in particular, a vinylidenefluoride-tetrafluoroethylene polymer (molar ratio=70-99:1-30), atetrafluoroethylene homopolymer, or a vinylidene fluoride homopolymer,

(5) an ethylene-tetrafluoroethylene- (hexafluoropropylene,3,3,3-trifluoropropylene-1, 2-trifluoromethyl-3,3,3-trifluoropropylene-1or perfluoro (C₁-C₃ alkyl vinyl ether)) polymer (molarratio=40-60:60-40:0-30),

(6) a polymer of tetrafluoroethylene and a fluorine-containing vinylcompound of the formula:

CF₂═CF—Rf

wherein Rf is a trifluoroalkyl group or a group of the formula: —ORf₁ inwhich Rf₁ is a C₁-C₅ perfluoroalkyl group (molar ratio=85-99.7:0.3-15),

(7) a vinylidene fluoride-tetrafluoroethylene-chlorotrifluoroethylenepolymer (molar ratio=50-99:0-30:1-20), and

(8) a vinylidene fluoride-tetrafluoroethylene-hexafluoropropylenepolymer (molar ratio=60-99:0-30:1-10), wherein a weight ratio of theelastomeric polymer chain segment(s) to the non-elastomeric polymerchain segment is 95-100:0-5 (both ends exclusive).

As the content of the non-elastomeric polymer chain segment mayincrease, the effects to solve the problems in curing and moldingimproves, while the properties of cured products worsen. Thus, thecontent of the non-elastomeric polymer chain segment is less than 5% byweight, preferably from 0.5 to less than 5% by weight, more preferablyfrom 0.5 to 3% by weight.

A typical chemical structure of a fluorine-containing block polymer isrepresented by, for example, the following formula:

Q[(A—B— . . . )I]_(n)

wherein Q is a residue of an iodide compound from which an iodine atomis removed, and A, B, . . . represent polymer chain segments, at leastone of which is a fluorine-containing polymer segment, I is an iodineatom removed from the iodide compound, and n is the number of bondingsites of Q and essentially consists of a chain comprising at least twopolymer chain segments, an iodine atom, and a residue derived from aniodide compound by removing at least one iodine atom therefrom.

Such at least two polymer chain segments are different from respectiveadjacent polymer chain segments (for example, segments having differentstructures or compositions of monomer units which constitute thesegments), and at least one of them is a fluorine-containing polymerchain segment, and they comprise at least one hard segment and at leastone soft segment. Preferably, at least one polymer chain segment has amolecular weight of at least 30,000, and one in a telomer range isexcluded.

A residue derived from an iodide compound by removing at least oneiodine atom therefrom may have a substituent, which is emanated from amonomer constituting the polymer chain segments or the iodide compound,when the iodide compound has a polymerizable double bond. In general,such a fluorine-containing block polymer contains 0.001 to 10% by weightof iodine atoms.

(b) An organic peroxide used according to the present invention may beany known organic peroxide which can liberate a peroxide radical at acuring temperature. Preferable examples of organic peroxides includedi-tert.-butyl peroxide, dicumyl peroxide,2,5-dimethyl-2,5-di(benzoylperoxy)hexane,2,5-dimethyl-2,5-di(tert.-butylperoxy)hexane, etc.

The content of an organic peroxide is usually from 0.05 to 10 parts byweight, preferably from 0.3 to 5 parts by weight, more preferably from 1to 5 parts by weight, per 100 parts by weight of the fluorine-containingblock polymer (a).

When the content of an organic peroxide is less than 0.05 part byweight, the fluorine-containing block polymer (a) may not sufficientlybe cured. When the content of an organic peroxide exceeds 10 parts byweight, the properties of cured products may deteriorate.

(c) A polyfunctional co-crosslinking agent used according to the presentinvention may be any known polyfunctional co-crosslinking agent which isused in combination with an organic peroxide in the peroxide-curing.Preferred examples of co-crosslinking agents include triallyl cyanurate,trimetharyl isocyanurate, triallyl isocyanurate, triacrylformal,triallyl trimellitate, N,N′-m-phenylenebismaleimide, diallyl phthalate,tatrallyl terephthalamide, tris(diallylamine)-S-triazine, triallylphosphite, N,N-diallylacrylamide, etc.

The content of a polyfunctional co-crosslinking agent is usually from0.1 to 10 parts by weight, preferably from 0.5 to 8 parts by weight,more preferably from 0.5 to 5 parts by weight, per 100 parts by weightof the fluorine-containing block polymer (a).

When the content of a polyfunctional co-crosslinking agent is less than0.1 part by weight, the fluorine-containing block polymer (a) may notsufficiently be cured. When the content of a polyfunctionalco-crosslinking agent exceeds 10 parts by weight, the elongation ofcured products may decrease.

The fluorine-containing elastomer composition of the present inventionmay optionally contain conventional additives which are compounded influorine-containing elastomer compositions, for example, fillers,processing aids, plasticizers, colorants, etc. It is possible tocompound one or more conventional curing agents and curing accelerators,which are di different from the above components (b) and (c).Furthermore, known fluororubbers may be compounded in thefluorine-containing elastomer composition of the present inventioninsofar as the effects of the present invention are not impaired.

The fluorine-containing elastomer composition of the present inventioncan be prepared by mixing the above components with any processingapparatus for rubbers, for example, open rolls, Banbury mixers,kneaders, etc.

The fluorine-containing elastomer composition of the present inventioncan be cured under conventional curing conditions which are employed tocure rubbers. For example, the composition is charged in a mold andpress cured under pressure at a temperature of 120 to 200° C. for 1 to60 minutes, and then oven cured in a furnace at a temperature of 120 to250° C. for 0 to 48 hours. Thus, a cured (vulcanized) rubber isobtained.

EXAMPLES

Now, the present invention will be explained in detail by the followingexamples, which do not limit the scope of the invention in any way.

The abbreviations used in the examples have the following meanings:

VdF: Vinylidene fluoride

HFP: Hexafluoropropylene

TFE: Tetrafluoroethylene

APS: Ammonium persulfate

Synthesis of Fluiorine-Containing Block Polymers

1) Synthesis of a fluorine-containing block polymer used in ComparativeExample 1

Pure water (3,000 ml) and ammonium perfluorooctanoate (6 g) were chargedin a 6,000 ml pressure reactor. After thoroughly replacing the internalatmosphere of the reactor with pure nitrogen gas, a mixed gas of TFE,VdF and HFP (molar ratio of 11:20:69) was injected at 80° C. whilestirring to pressurize the reactor to 15 kgf/cm²G. Then, a 0.4 wt. %aqueous solution of APS (10 ml) was charged under pressure. As soon asthe APS solution was added, a polymerization reaction was initiated, andthe pressure dropped. When the pressure dropped to 14 kgf/cm²G, a mixedgas of TFE, VdF and HFP (molar ratio of 20:50:30) (supplement chargegas) was injected to repressurize the reactor to 15 kgf/cm²G.Thereafter, the polymerization was continued while maintaining theinternal pressure in the range between 14 and 15 kgf/cm²G by the aboveprocedures.

In the course of the polymerization, when the total amount of thesupplement charge gas reached 25 g, ICF₂CF₂CF₂CF₂I (4.6 g) was charged,and the 0.4 wt. % aqueous solution of APS (each 10 ml) was injectedevery 3 hours from the initiation of the polymerization.

When the total amount of the supplement charge gas reached 1,000 g, thetemperature was lowered and the pressure was released to terminate thepolymerization. The polymerization time was 18 hours.

The solid content of the obtained dispersion was 24.8% by weight.

The dispersion was coagulated with a line mixer exerting strong shearforce, and the coagulated material was washed with water and dried.Thus, a fluorine-containing elastomer having a Mooney viscosityML₁₊₁₀100° C. of 50 (990 g) was obtained.

2) Synthesis of fluorine-containing block polymer (i)

After the dispersion was prepared in the same manner as in the abovesynthesis 1), the internal atmosphere of the reactor was thoroughlyreplaced with pure nitrogen gas. Then, the reactor was pressurized to 1kgf/cm²G with TFE, and a 0.4 wt. % aqueous solution of APS (10 ml) wasinjected. As soon as the APS solution was added, a polymerizationreaction was initiated, and the pressure dropped. When the pressuredropped to 0 kgf/cm²G, TFE was injected to repressurize the reactor to 1kgf/cm²G. When the supplemented amount of TFE reached 25 g in total andthe pressure was increased to 1 kgf/cm²G, which was the same as thepressure at the time of polymerization initiation, the temperature waslowered and the pressure was released to terminate the polymerization.The polymerization time was 2 hours and 30 minutes.

The solid content of the obtained dispersion was 25.5% by weight.

The dispersion was coagulated, washed with and dried in the same manneras in the synthesis 1). The obtained fluorine-containing elastomer had aMooney viscosity ML₁₊₁₀100° C. of 80, and the yield was 1,020 g.

The content of the PTFE segment corresponded to 2.5% by weight(calculated from the charged amount).

3) Synthesis of fluorine-containing block polymer (ii)

After the dispersion was prepared in the same manner as in the abovesynthesis 1), the internal atmosphere of the reactor was thoroughlyreplaced with pure nitrogengas. Then, the reactor was pressurized to 15kgf/cm²G with VdF, and a 0.4 wt. % aqueous solution of APS (10 ml) wasinjected. As soon as the APS solution was added, a polymerizationreaction was initiated, and the pressure dropped. When the pressuredropped to 14 kgf/cm²G, VdF was injected to repressurize the reactor to15 kgf/cm²G. When the supplemented amount of VdF-reached 45 g, thetemperature was lowered and the pressure was released to terminate thepolymerization. The polymerization time was 1 hour and 10 minutes.

The solid content of the obtained dispersion was 25.8% by weight.

The dispersion was coagulated, washed with and dried in the same manneras in the synthesis 1). The obtained fluorine-containing elastomer had aMooney viscosity ML₁₊₁₀100° C. of 95, and the yield of the rubber was1,020 g.

The content of the PVdF segment corresponded to 4.5% by weight.

4) Synthesis of fluorine-containing block polymer (iii) used inComparative Example 2

After the dispersion was prepared in the same manner as in the abovesynthesis 1), the internal atmosphere of the reactor was thoroughlyreplaced with pure nitrogen gas. Then, the reactor was pressurized to 1kgf/cm²G with TFE, and a 0.4 wt. % aqueous solution of APS (10 ml) wasinjected. As soon as the APS solution was added, a polymerizationreaction was initiated, and the pressure dropped. When the-pressuredropped to 0 kgf/cm²G, TFE was injected to repressurize the reactor to 1kgf/cm²G. The pressurizing and supplemental charge of TFE were repeated.When the supplemented amount of TFE reached 110 g in total, thetemperature was lowered and the pressure was released to terminate thepolymerization. The polymerization time was 8 hours and 30 minutes. Inthis case, a 0.4 wt. % aqueous solution of APS (each 10 ml) was injectedafter 3 hours and 6 hours from the initiation of the polymerization.

The solid content of the obtained dispersion was 27.0% by weight.

The dispersion was coagulated, washed with and dried in the same manneras in the synthesis 1). The obtained fluorine-containing elastomer had aMooney viscosity ML₁₊₁₀100° C. of 104, and the yield was 1,101 g.

The content of the PTFE segment corresponded to 10.0% by weight.

5) Synthesis of fluorine-containing block polymer used in ComparativeExample 3

Pure water (3,000 ml) and ammonium perfluorooctanoate (6 g) were chargedin a 6,000 ml pressure reactor. After thoroughly replacing the internalatmosphere of the reactor with pure nitrogen gas, a mixed gas of VdF andHFP (molar ratio of 50:50) was injected to pressurize the reactor to 15kgf/cm²G. Then, a 0.4 wt. % aqueous solution of APS (10 ml) was chargedunder pressure. As soon as the APS solution was added, a polymerizationreaction was initiated, and the pressure dropped. When the pressuredropped to 14 kgf/cm²G, a mixed gas of VdF and HFP (molar ratio of78:22) (supplement charge gas) was injected to repressurize the reactorto 15 kgf/cm²G. Thereafter, the polymerization was continued whilemaintaining the internal pressure in the range between 14 and 15kgf/cm²G by the above procedures.

In the course of the polymerization, when the total amount of thesupplement charge gas reached 25 g, ICF₂CF₂CF₂CF₂I (4.6 g) was charged,and the 0.4 wt. % aqueous solution of APS (each 10 ml) was injectedevery. 3 hours from-the initiation of the polymerization.

When the total amount of the supplement charge gas reached 1,000 g, thetemperature was lowered and the pressure was released to-terminate thepolymerization. The polymerization time was 28 hour.

The solid content of the obtained dispersion was 24.7% by weight.

The dispersion was coagulated with a line mixer exerting strong shearforce, and the coagulated material was washed with water and dried.Thus, a fluorine-containing elastomer having a Mooney viscosityML₁₊₁₀100° C. of 66 (993 g) was obtained.

6) Synthesis of fluorine-containing block polymer (iv)

After the dispersion was prepared in the same manner as in the abovesynthesis 5), the internal atmosphere of the reactor was thoroughlyreplaced with pure nitrogen gas. Then, the reactor was pressurized to 1kgf/cm²G with TFE, and a 0.4 wt. % aqueous solution of APS (10 ml) wasinjected. As soon as the APS solution was added, a polymerizationreaction was initiated, and the pressure dropped. When the pressuredropped to 0 kgf/cm²G, TFE was injected to repressurize the reactor to 1kgf/cm²G. When the supplemented amount of TFE reached 45 g in total andthe pressure was increased to 1 kgf/cm²G, which was the same as thepressure at the time of polymerization initiation, the temperature waslowered and the pressure was released to terminate the polymerization.The polymerization time was 5 hours.

The solid content of the obtained dispersion was 25.7% by weight.

The dispersion was coagulated, washed with and dried in the same manneras in the synthesis 5). The obtained fluorine-containing elastomer had aMooney viscosity ML₁₊₁₀100° C. of 86, and the yield of the rubber was1,039 g.

The content of the PTFE segment corresponded to 4.5% by weight.

Examples 1-3 and Comparative Examples 1-3

The components shown in Table 1 were successively added and well kneadedon rubber rolls, and the compound was kept standing on the rollsovernight to age it. Then, the compound was again kneaded on the rolls,and press cured and oven cured under the conditions specified inTable 1. Thus, a sheet-form cured rubber and O-rings were obtained.

Physical properties and molding properties in a mold of the obtainedsheet-form cured rubber and O-rings were measured or evaluated asfollows:

a) The hardness of a cured material was measured according to JIS K6253, Type A, while 100% tensile stress, tensile strength and elongationwere measured according to JIS K 6301.

b) Compression set was evaluated using P-24 dynamic O-ring, which isdefined in JIS B 2401, according to JIS K 6301 under the conditions-of200° C. ×700 hours and 25% compression.

c) Molding properties in a mold was evaluated with respect to thefollowing two items by press curing a composition three shots (one shot:170° C., 4 minutes) in armold for P-8 dynamic O-rings (65 pieces) whichis defined in JIS B 2401.

When the mold release property was evaluated, a rubber with whichcomponents were compounded, was kept standing at 40° C. for 7 days, andthen press cured. This assumed that triallyl isocyanurate and2,5-dimethyl-2,5-di(tert.-butylperoxy)hexane bleed on the surface of thecompound, and thus the compound has low molding properties insummertime. The used mold was made of iron the surface of which isplated with hard chromium.

Evaluated items:

1. The number of O-rings (total of three shots) in which the body of theO-ring adhered to the mold, and a burr was broken at a pinch-off part,when the cured products were removed from the mold.

2. Whether a part of a burr contaminates a mold when the cured productsare removed from the mold.

TABLE 1 Ex. 1 Ex. 2 C. Ex. 1 C. Ex. 2 Ex. 3 Ex. 4 Component (wt. parts)Fluororubber F-cont. F-cont. Polymer F-cont. F-cont. Polymer block blockobtained in block block obtained in polymer polymer synthesis 1) polymerpolymer synthesis 5) (i) (ii) 100 (iii) (iv) 100 100 100 100 100 MTcarbon black 20 20 20 20 20 20 Triallylisocyanurate 4 4 4 4 4 42,5-Dimethyl-2,5- 1.5 1.5 1.5 1.5 1.5 1.5 di(tert.-butyloxy)- hexaneCuring conditions Press cure 160° C. × 160° C. × 160° C. × 160° C. ×160° C. × 160° C. × 10 min. 10 min. 10 min. 10 min. 10 min. 10 min. Ovencure 18.0° C. × 180° C. × 180° C. × 180° C. × 180° C. × 180° C. × 4 hrs.4 hrs. 4 hrs. 4 hrs. 4 hrs. 4 hrs. Properties of cured product 100%Tensile stress 45 52 38 147 41 23 (kgf/cm²) Tensile strength 274 269 251303 269 239 (kgf/cm²) Elongation (%) 310 310 340 295 460 470 Hardness 7474 72 86 74 67 Compression set (%) 23 23 23 26 26 27 Molding propertiesin mold Number of O-rings 0 10 50 0 0 1 remained on mold Contaminationof No No No No No Yes mold without a part of burr

In comparison with the known peroxide-curable fluororubber ofComparative Example 1 (a vinylidenefluoride-tetrafluoroethylene-hexafluoropropylene elastomer), thefluorine-containing block polymers of Examples 1 and 2 (a vinylidenefluoride-tetrafluoroethylene-hexafluoropropylene elastomer containing2.5% by weight of a PTFE segment and 4.5% by weight of a PVdF segment,respectively) reduced the number of O-rings remained on a mold, andcaused less contamination with a part of a burr. Thus, the blockpolymers of Examples 1 and 2 had better molding properties than thefluororubber of Comparative Example 1.

The fluorine-containing block polymer of Comparative Example 2 had acontent of a PTFE segment of 10% by weight. Thus, it had good moldingproperties, but the hardness of a cured product increased, and thecompression set was not good, because of the high content of the PTFEsegment.

In comparison with the known peroxide-curable fluororubber ofComparative Example 3 (a vinylidene fluoride-hexafluoropropyleneelastomer), the fluorine-containing block polymer of Example 3 (avinylidene fluoride-hexafluoropropylene elastomer containing 4.5% byweight of a PTFE segment) caused less contamination with a part of aburr. Thus, the block polymer of Example 3 had better molding propertiesthan the fluororubber of Comparative Example 3.

What is claimed is:
 1. A peroxide-curable fluorine-containing elastomercomposition comprising (a) 100 parts by weight of a fluorine-containingblock polymer comprising at least one elastomeric polymer chain segmentand at least one non-elastomeric polymer chain segment in which thecontent of said non-elastomeric polymer chain segment is less than 5% byweight, (b) 0.05 to 10 parts by weight of an organic peroxide, and (c)0.1 to 10 parts by weight of a polyfunctional co-crosslinking agent. 2.The peroxide-curable fluorine-containing elastomer composition accordingto claim 1, wherein said elastomeric polymer segment of saidfluorine-containing block polymer is at least one polymer segmentselected from the group consisting of a vinylidenefluoride-tetrafluoroethylene-hexafluoropropylene polymer, a vinylidenefluoride-hexafluoropropylene polymer and atetrafluoroethylene-perfluoroalkyl vinyl ether polymer, and saidnon-elastomeric polymer segment is polytetrafluoroethylene orpolyvinylidene fluoride.
 3. The peroxide-curable fluorine-containingelastomer composition according to claim 1, wherein the content of saidnon-elastomeric polymer chain segment is from 0.5 to 3% by weight. 4.The peroxide-curable fluorine-containing elastomer composition accordingto claim 1, wherein the organic peroxide is present in an amount of from1 to 5 parts by weight based on 100 parts by weight of thefluorine-containing block polymer (a).
 5. The peroxide-curablefluorine-containing elastomer composition according to claim 1, whereinthe polyfunctional co-crosslinking agent is present in an amount of from0.5 to 5 parts by weight based on 100 parts by weight of thefluorine-containing block polymer (a).
 6. The peroxide-curablefluorine-containing elastomer composition according to claim 1, whereinthe organic peroxide is selected from the group consisting ofdi-tert.-butyl peroxide, dicumyl peroxide,2,5-dimethyl-2,5-di(benzolyperoxy)hexane, and2,5-dimethyl-2,5-di(tert.-butylperoxy)hexane.
 7. The peroxide-curablefluorine-containing elastomer composition according to claim 2, whereinthe organic peroxide is selected from the group consisting ofdi-tert.-butyl peroxide, dicumyl peroxide,2,5-dimethyl-2,5-di(benzolyperoxy)hexane, and2,5-dimethyl-2,5-di(tert.-butylperoxy)hexane.
 8. The peroxide-curablefluorine-containing elastomer composition according to claim 1, whereinthe polyfunctional co-crosslinking agent is selected from the groupconsisting of triallyl cyanurate, trimetharyl isocyanurate, triallylisocyanurate, triacrylformal, triallyl trimellitate,N,N′-m-phenylenebismaleimide, diallyl phthalate, tetrallylterephthalamide, tris(diallylamine)-S-triazine, triallyl phosphite, andN,N-diallylacrylamide.
 9. The peroxide-curable fluorine-containingelastomer composition according to claim 2, wherein the polyfunctionalco-crosslinking agent is selected from the group consisting of triallylcyanurate, trimetharyl isocyanurate, triallyl isocyanurate,triacrylformal, triallyl trimellitate, N,N′-m-phenylenebismaleimide,diallyl phthalate, tetrallyl terephthalamide,tris(diallylamine)-S-triazine, triallyl phosphite, andN,N-diallylacrylamide.
 10. The peroxide-curable fluorine-containingelastomer composition according to claim 7, wherein the polyfunctionalco-crosslinking agent is selected from the group consisting of triallylcyanurate, trimetharyl isocyanurate, triallyl isocyanurate,triacrylformal, triallyl trimellitate, N,N′-m-phenylenebismaleimide,diallyl phthalate, tetrallyl terephthalamide,tris(diallylamine)-S-triazine, triallyl phosphite, andN,N-diallylacrylamide.